Burial history and thermal maturity of Mesozoic rocks of the Dolomites, Northern Italy

Abstract

Thermal maturity analyses provide a valid basis to reconstruct the burial and temperature history of sedimentary rocks. In combination with computer based modeling it is possible to quantify former overburden and erosional thickness. This study was carried out to analyze thermal maturity and elaborate the burial history of Permo-Mesozoic sediments of the Western Dolomites, Northern Italy. Based on their location in the non-metamorphosed Southern Alps of the Alpine realm, the Dolomites are well suited to study thermal maturation in a complex orogenic setting. Determination of maturity was carried out by vitrinite reflectance and solid bitumen reflectance measurements on a large number of samples as well as by Rock–Eval pyrolysis. Vitrinite reflectance commonly ranges between 0.5 % VR_r for Triassic and 0.76 % VR_r for Permian sediments allowing an analysis of the lateral and vertical maturation patterns. Several 1D basin models were developed and calibrated by maturity measurements. Results show that deepest burial probably occurred at the end of Cretaceous times, followed by erosion of about 1800 ± 200 m. The estimated heat flows were moderate with 75 mW/m² for Jurassic rifting, 46 mW/m² for times of deepest burial and 41 mW/m² for present day.

1 Introduction

In sedimentary basin research, knowledge of the burial history is the key to reconstruct basin evolution.

With modern numerical modeling techniques it is possible to simulate and quantify the burial history of sedimentary basins (e.g. Bruns et al. 2013). As a prerequisite for numerical simulation, a good understanding of the geology, including lithology, thickness and stratigraphy is needed. Moreover, data representing the thermal history of the basin is required to calibrate and verify the simulated burial trends. Dispersed organic matter in sedimentary basins reacts strongly and irreversibly to temperature increase (Bostick and Foster 1975; Scheidt and Littke 1989). Thus, it is well suited for petrological and geochemical analyses revealing parameters that represent the maximum temperatures reached upon deep burial. A common parameter to analyze thermal maturity is the reflectance of dispersed organic matter, such as vitrinite or solid bitumen particles (e.g. Nöth et al. 2001; Schoenherr et al. 2007; Littke et al. 2012).

Within the Alps the Dolomites are characterized by a geological situation which favors this type of analysis of thermal and burial history. Deposits of various stratigraphic formations are still present in their original lateral continuity and intense folding, nappe thrusting and metamorphism, widespread in most alpine regions, did not occur in the area of the Dolomites (Doglioni 1987; Bosellini et al. 2003). This geological situation is clearly present in the western Dolomites, the study area of this work.

In the western Dolomites a Mesozoic sedimentary succession is underlain by thick layers of sedimentary rocks and volcanics of Permian age. This volcanic sheet prevented a stronger deformation of the area. The overlying Mesozoic column is represented by 2500 to 3000 m of sedimentary rocks, mainly carbonates with interlayered shales (Bosellini 1998; Brandner et al. 2011).

In areas affected by uplift events during geological history the eroded thicknesses can be reconstructed on the basis of burial history modeling. For some alpine areas such uplift and erosional histories have been studied by analysis of apatite fission tracks (Central Alps: Bernet et al. 2001; Appenines: Dunkl et al. 2001; Venetian foreland: Zattin et al. 1990) or other maturity parameters, e.g. in Austria (e.g. Sachsenhofer 1992; Rantitsch et al. 2005), Switzerland (e.g. Suchy et al. 1997; Mählmann and Giger 2012) and Slovenia (Rainer et al. 2009). In the Dolomites only a few studies, partly published in German, focused on reconstruction of thermal maturity (Emmerich et al. 2005, 2008; Tscherny 2006; Tscherny et al. 2011; Blendinger et al. 2013). Tscherny et al. (2011) performed numerical modeling for the area east of the city of Corvara (Fig. 1) and estimated eroded thicknesses in the order of 1700–2400 m.

Fig. 1

Geological map of the Dolomites. The Dolomites are bordered by the Periadriatic Lineament (PAL) in the north and the Suganer Line in the south. Highlighted are the study areas of this and former vitrinite reflectance studies [Map after Brandner (1980) and Bigi et al. (1990), modified according to Tscherny (2006)]

The objective of the present study is a reconstruction of the burial and uplift history of Mesozoic sediments and the evaluation of the eroded thicknesses of the Cretaceous layers for the northwestern Dolomites and a comparison with results from previous studies on other parts of the Dolomites. Three representative sections were chosen to evaluate the burial history by application of the PetroMod 1D software package (Schlumberger; Version 2012). Thermal maturity was analyzed by vitrinite and solid bitumen reflectance measurements to determine maximum burial depth. These data are essential for calibration of the modelled burial history. Additionally, elemental analysis and Rock–Eval pyrolysis were carried out in order to obtain information on petroleum generation and depositional conditions. This data set was interpreted in the context of previous studies on the evolution of the western Dolomites.

2 Geological setting and stratigraphy

The regional stratigraphy of the study area in the Dolomites (Fig. 1) comprises Permian to Cretaceous, but mainly Triassic rock units, underlain by a deformed Paleozoic basement metamorphosed during the Variscan Orogeny (Fig. 2, Bosellini et al. 2003). Initial rifting in the Early Permian produced thick volcanic layers (Athesian Volcanic Group or Bozen Porphyry Plateau) (Marocchi et al. 2008).

Fig. 2

Simplified stratigraphy of the Dolomites in the Val Gardena area. Listed ages and stratigraphies are used in the numerical model. Modified after Tscherny et al. (2011). Compiled from Bosellini (1998), Keim et al. (2001) and Bosellini et al. (2003). Ages from Mundil et al. (2010) and Ogg et al. (2014)

The sedimentary succession of the study area started in the Permian with the Val Gardena Sandstone (Grödner Sandstein), representing red beds of a semi-arid paleo setting, covered by shallow marine evaporites and carbonates (Bellerophon Formation) (Massari and Neri 1997; Bosellini et al. 2003) towards the end of the Permian. They are unconformably covered by shallow-water carbonates and interbedded terrigenous deposits of the Lower Triassic Werfen Formation. After uplift and local deposition of terrestrial clastic sediments (Perez Fm., locally used unit, as Voltago Conglomerate and Richthofen Conglomerate are not clearly distinguishable in the study area) renewed transgression in the western part resulted in lagoonal deposits comprised of terrigenous siliciclastics and carbonates of the Morbiac Fm. Then first carbonate platforms developed in shallow marine areas (Contrin Fm.).

During the Late Anisian a fast subsidence pulse was associated with platform drowning and contemporaneous quick upward aggradation (Stefani et al. 2010). Thereby, huge carbonate platforms (Sciliar/Schlern Fm.) were produced, with deep basin sediments deposited in between (Buchenstein Fm.). Platform examples are the Marmolata and Catenaccio (Rosengarten) complexes.

During the tectono-magmatic event of Ladinian age, some platforms were penetrated by dykes and large volcanic collapses occurred (Bosellini et al. 2003). Volcanic, erosional debris with interlayered, calcareous detritus was subsequently deposited in the basin facies (Wengen Fm., Fig. 2) (Keim 2008). The San Cassiano Formation (Keim 2008) represents a continuous sedimentation of calcareous basin fill lacking any volcanic detritus. As basinal platform the Cassian Dolomite developed on top of it, overlain by shallow marine to continental depostits of the Raibl Group (Stefani et al. 2010).

Subsequently, a massive carbonate platform (Dolomia Principale) developed due to slow but continuous subsidence (Hardie and Goldhammer 1986; Blendinger 2004; Emmerich et al. 2005).

Towards the end of the Norian (Late Triassic), the dry, arid climate changed to wet climate (Bosellini 1998; Berra et al. 2010). A slow plateau subsidence, resulted in the change from the deposition of the shallow marine Calcari Grigi deposition to deeper-water sedimentation of slumps and redeposited pelagic sediments of the Ammonitico Rosso (Bertotti et al. 1993; Masetti et al. 2012). The present day thicknesses of the Calcari Grigi and the Ammonitico Rosso are varying in the study area: East of the Val Badia Triassic deposits are followed without any hiatus by Jurassic sediments. However, the Jurassic is completely missing or preserved only as very thin and partly eroded layer on the western site of the valley. In the latter area the Cretaceous Puez-Marls follow unconformably above the Triassic Dolomia Principale (Tscherny 2006).

With the opening of the Ligurian-Piedmontese ocean in the Middle to Upper Jurassic a new system of rifting evolved (Bertotti et al. 1993) and the area of the Eastern Dolomites became a passive continental margin (Carminati et al. 2010). Nevertheless, the area was affected by ongoing subsidence during the Upper Jurassic and Cretaceous (Winterer and Bosellini 1981). At the end of the Jurassic the depositional system changed to a more pelagic-clastic one with calcarenites and mud turbidites (Carminati et al. 2010).

The stratigraphy of the Cretaceous is complex, but almost all the Cretaceous is documented at different places of the Dolomites. In the summit region of the Gardenaccia, only the lower Cretaceous is preserved (Maiolica and Puez Marls) above very thin Jurassic units (Stock 1994). However, in the Sella group Cretaceous sediments follow conformably above the Ammonitico Rosso or above an erosional surface within the Dolomia Principale. In the northern Dolomites, in the Fanes-Sennes area, the Upper Cretaceous units are very well known and the succession spans from Maiolica (Tithonian to Barremian) to Scaglia Rossa (Antruilles Fm, Turonian–Maastrichtian).

In the Val Gardena area the only Cretaceous sediments preserved are the above mentioned of the Puez-Marls (Bosellini 1998; Lukeneder and Aspmair 2006; Lukeneder 2010). All other Cretaceous sediments were eroded during the Late Cretaceous and/or Tertiary (Fig. 2) due to the uplift of the Alps (Doglioni 1984, 1992; Carminati and Doglioni 2012). Apart from the Mont-Parei-Conglomerate, Tertiary deposits are not preserved in the study area and the Quaternary is represented only by thin sediment fillings of Alpine valleys.

3 Samples and methods

Sample coverage of maturity measurements by selected previous publications (Tscherny 2006; Emmerich et al. 2008; Blendinger et al. 2013) is shown in Fig. 1. In combination with the previously assembled data set, an improved database was achieved as a solid basis for numerical basin modeling.

Based on fieldwork and literature information, the following stratigraphic layers were identified as potentially vitrinite-bearing: Val Gardena Fm., Wengen Fm., Heiligkreuz Fm., Moena Fm. (Bosellini 1998; Tscherny 2006), Buchenstein Fm., Morbiac Fm. and the Upper Bellerophon Fm. (Bosellini et al. 2003). Among those, the two first listed formations were classified as “good” and the remaining ones as “medium”-quality using the quality criteria of vitrinite reflectance measurements (Tscherny 2006). He classified samples as “good-quality” provided they allowed the identification of more than 30 vitrinite particles for reflectance measurements, whereas samples with fewer vitrinite particles were termed “medium-quality”. In each of these formations the interlayered, laminated and marly layers are best suited. Locations close to intrusions and volcanic dykes were avoided in order to exclude local thermal overprinting of organic matter maturity.

These considerations led to the selection of two areas for fieldwork and sampling. The first area is the Gardena Valley (ital. Val Gardena.) between St. Ullrich (Ortisei/Urtijei) and Wolkenstein (Selva Gardena), approximately 25 km northeast of the city of Bozen. The second area of interest is the Frötschbach area, approximately 3 km southeast of the village of Seis (see "Appendix Table 4" for sample coordinates).

In the Gardena Valley, 43 samples were collected from three sections. In all of them the stratigraphic units from Bellerophon to Buchenstein are cropping out. Mainly the Buchenstein Formation, and the Bulla Member of the Upper Bellerophon Fm. were sampled.

Apart from these three sections the following additional Buchenstein Fm. samples were collected: four from the Seceda, one from the Würzjoch and 12 from the eastern continuation of the Gardena Valley, north of the city of Colfosco.

In the Frötschbach area two sections were sampled; one in the valley of the Frommerbach, where the complete stratigraphic sequence from the Perez Fm. to the Wengen Fm. is cropping out. Another section was sampled on the slopes of the Frötschbach valley itself.

A total number of 153 samples was collected from outcrop sections, whereby fresh and visibly unweathered material was chosen. For characterization of elemental composition 37 dark-colored samples were selected for analysis of total organic and inorganic carbon (TOC, TIC). From this group 23 were also analyzed for total sulfur content (TS). For elemental analyses, 100 mg of each pulverized sample was combusted using a liquiTOC II analyzer to determine the total organic and inorganic carbon content. The instrument is using a temperature ramp method, which enables a direct determination of TOC and TIC without prior acidification of the samples (heating in two steps: 300 °C/min to 550 °C and held for 600 s, than raised to 1000 °C and held for 400 s). Under constant oxygen supply the organic carbon (first step) and the inorganic, carbonate carbon (second step) will generate CO₂, which is analyzed by a non-dispersed infrared detector (NDIR: detection limit 10 ppm, rel. TOC error 0.6 %, TIC error 1.7 %).

Based on this data CaCO₃ and CaMg(CO₃)₂ contents are calculated for all samples as follows:

$$CaCO_{3} = TIC \cdot 8.333$$

$$CaMg(CO_{3} )_{2} = TIC \cdot 7.667$$

Sulfur (total sulfur, TS) was measured on a LECO S200. After covering 100 mg of sample with Fluorhib and Fe-cuttings, it is combusted under constant oxygen supply in a high frequency induction furnace at 1000 °C. During this procedure sulfur is oxidized to SO₂. The generated gases are flashed by the carrier gas oxygen through a water trap to remove water vapor and then to a SO₂-specific infrared cell to measure the total sulfur content (detection limit 20 ppm, relative error <5 %).

Thermal maturity was characterized by vitrinite and solid bitumen reflectance measurements. Samples selected on the basis of their color and TOC content were prepared for organic petrological analysis. Samples cut perpendicular to bedding were impregnated with a mixture of epoxy-resin (Scandiplex A) and a hardener (Scandiplex B). During the exothermal hardening maximum temperatures did not exceed 36 °C. Each block was polished on one side with successively finer abrasives (320, 400, 600 mm; see Littke et al. 2012; Sachse et al. 2012 for more details).

Reflectance measurements were performed on an incident light microscope Zeiss Axioplan with an oil immersion lens of 50-times magnification under monochromatic light with a wavelength of 546 nm. Oil with known refractive index was used to avoid undesired diffusion reflectance (Taylor et al. 1998). The microscope was calibrated with an yttrium–aluminum–garnet-standard (YAG) with a constant reflectance of 0.889 %; this standard can be used up to values of 1.3 % VR_r. According to Senftle et al. (1993) measurement of at least 30 vitrinite particles should be made to obtain a representative mean value. Due to a lack of vitrinites, this number was not reached for some of the samples.

Vitrinite reflectance data can be used to characterize a broad range of maturities, from early diagenesis (~0.2 % VR) up to low grades of metamorphism (~>4.0 % VR) in order to determine maximum burial depth (Sweeney and Burnham 1990; Barker and Pawlewicz 1994). As a first estimation, maximum temperatures of burial heating were calculated based on the following formula by Barker and Pawlewicz (1994):

$$T_{peak} = \frac{{\ln \left( {VR_{r} } \right) + 1.68}}{0.0124}$$

In many cases two maxima of vitrinite reflectance values are observed. In such cases, the lower reflectance values normally represent the autochthonous vitrinites since the higher maximum usually represents resedimented, allochthonous particles (Taylor et al. 1998). Solid bitumen reflectance (BR_r) can be measured if vitrinites are not observed, e.g. in pre-Devonian rock or rocks containing little terrestrial material. Measured values reflect thermal maturity and can be converted into vitrinite reflectance by the formula of Schoenherr et al. (2007):

$$VR_{r}\, \left( \% \right) = \frac{{\left( {BR_{r} + 0.2443} \right)}}{1.0495}$$

To obtain VR_r vs. depth trends from field data without expensive drilling campaigns pseudowells were created according to the method of Nöth et al. (2001). Therefore, sediments of different formations were sampled at the surface and measured for VR_r and BR_r. Results were extrapolated into a VR_r vs. pseudo-depth profile perpendicular to bedding.

Based on their TOC values 9 samples were selected for Rock–Eval analysis to characterize the quality of organic matter. 100 mg of each pulverized sample was treated according to Espitalié et al. (1985).

The temperature of the generated maximum S2 yield (T_max) was used as another maturity parameter. According to Hunt (1995) “the hydrogen index (S2 normalized to TOC) could be roughly correlated with the atomic H/C ratio and the oxygen index with the atomic O/C ratio of the van Krevelen diagram”. Thus, in a Hydrogen Index (HI) versus Oxygen Index (OI) diagram kerogen quality can be interpreted in a similar way as in the well-known van Krevelen plot. Due to problems with a strong mineral-matrix-effect in the case of low TOC values, only samples with TOC contents in excess of 0.5 wt% were selected for analyses (Saeed and Peters 1994).

To quantify maximum burial temperature and depth integrated basin modeling was performed for the two study areas with the PetroMod1D 2012.1 software package, developed by the Schlumberger Technology Center Aachen, Germany. The software uses a forward modeling, event-stepping approach. In this way, the simulation starts with the deposition of the oldest sediment layer and continues with decreasing geological age of the strata, event by event, until the present-day geometry of the sediment-fill is reached.

As a first step, a generalized present-day stratigraphic column of the studied sections needs to be defined, including lithologies, petrophysical data, depositional ages and thicknesses of all units. This so called conceptional model is based on a chronology of events, i.e. sedimentation, erosion, hiatus (non-deposition) and tectonic events (e.g., salt movement/doming, volcanism, thrusting).

Thermal boundary conditions are needed to define the overall thermal regime of the sedimentary basin over time. The two main input parameters are the basal heat flow [HF (mW/m²), lower thermal boundary] and the paleo water depth [PWD (m), upper boundary]. The PWD is used to calculate the sediment water interface temperature [SWIT (°C)] (Hantschel and Kauerauf 2009). Furthermore, internal radiogenic heat production is calculated based on sedimentary lithologies depending on their average abundance of uranium, thorium and potassium.

For each buried layer the software calculates vitrinite reflectance according to the EASY %R₀ method of Sweeney and Burnham (1990). This calculated VR value is compared with the measured VR value and the model is adjusted until a best fit of the VR vs. depth trend is achieved.

4 Results and discussion

4.1 Elemental analysis

Results of the elemental analyses are listed in Table 1 and plotted in Fig. 3. The samples show a wide range of TOC, even within a single formation (Fig. 3a). TOC values range from a shaly limestone with 2.3 % (13/696) to a red shale with 0.01 % (13/639). Most samples vary between 0.3 and 1.8 % TOC and a weak correlation of TOC and lithology is observed. Limestones with interbedded shales (shaly limestone) range between 1.1 (13/653) and 2.3 % TOC (13/696), whereas purer limestones are characterized by lower TOC values between 0.3 (13/660) and 0.8 % (13/677).

Table 1 CaCO₃ and CaMg(CO₃)₂ calculated from TIC

Fig. 3

Plots of analysis data: a TOC values of studied formations, b TS vs. TOC. Black line represents normal marine conditions (after Berner 1984), and c TOC vs. CaCO₃ contents

Calculated carbonate contents vary between 6.3 (13/699) and 115.7 % (13/745, based on calculation of calcite content). Data exceeding 100 % are explained by a significant contribution of dolomite rather than calcite (Table 1).

Measurement of total sulfur content (TS) allows to characterize the depositional environment (Fig. 3b, Berner 1984; Lückge et al. 1999, 2002). Values for total sulfur contents vary between nearly 0 and 0.95 wt%.

Figure 3b shows that most samples plot below the normal marine trend (Berner 1984). Two samples (13/666, 13/684) show much higher TS/TOC ratios. This can be explained by the influence of oxygen-free bottom waters and/or the occurrence of late diagenetic pyrite (FeS₂). The occurrence of pyrite was documented for all samples by microscopic examination. Low sulfur contents, e.g. within the Wengen or Perez Fm., may indicate the influence of weathering (Littke et al. 1991), which is not the case here, as the pyrite is fresh.

4.2 Organic petrology

Vitrinites are organic particles derived from woody tissues of higher land plants, mainly originating from lignin and cellulose of plant cell walls (Taylor et al. 1998). During burial and coalification chemical and physical changes take place, which are temperature dependent and irreversible. Especially, with increasing thermal maturation vitrinite becomes more aromatized (Schenk et al. 1990) and as a consequence its ability to reflect light increases. Therefore, vitrinite reflectance can be used to determine the thermal maturity of the organic matter in sedimentary rocks. In most cases vitrinite reflectance increases exponentially with a linear increase of temperature (amongst others: Ting 1973; Saxby et al. 1986). Due to the fact that coalification is irreversible, the maximum temperature a rock has experienced is archived in the reflectance of vitrinites in sedimentary rocks.

For 46 samples reflectance was measured: 42 of them contain vitrinites and four solid bitumen ("Appendix Table 4"). In four samples neither vitrinites nor solid bitumen has been detected (13/643, 13/688, 13/710 and 13/711). New obtained data (this study) were compared to literature data from other parts of the Dolomites (Buggisch 1978; Schulz and Fuchs 1991; Bielefeld 1998; Tscherny 2006; Emmerich et al. 2008; Blendinger et al. 2013). Thus, a total number of 75 samples with vitrinite reflectance values is available ("Appendix Table 4"). They range from immature (e.g. from 0.37 % VRr to 0.60 % VRr) to postmature (up to 1.75 % VR_r, 13/681) stages (classification according to Peters et al. 2005). Most data represent samples from within the oil window, below or at peak oil generation (41 samples, 0.60–0.90 % VR_r). Only six samples are characterized as immature and 24 samples are interpreted as late mature (0.90–1.35 VR_r). The latter ones are interpreted as overprinted by Ladinian volcanic intrusions and are marked (^v) in the table of the Appendix. Also, samples are marked (^std) for which the variation of measured values, i.e. the standard deviation, is very high, resulting in less reliable information as compared to the remaining samples.

The whole area of the Gardena Valley/Val Gardena shows mature vitrinites (0.61–0.75 % VR_r) at all sampled locations with a more or less normally increasing depth trend (VR vs. depth trend established by a plot of measured VR versus the stratigraphic position of each sample). The study area around the Frötsch- and Frommerbach indicates the same trend with mature to late mature samples (0.50–0.92 % VR_r). Similar results are observed in the Colfosco area with mature to late mature range (0.73–0.96 % VR_r).

The VR data of the Buchenstein Fm. indicate slightly increasing values from east to west and also from north to south.

4.3 Rock–Eval pyrolysis

Based on their TOC values, 9 samples were selected for Rock–Eval pyrolysis. Table 2 shows the results. According to Hunt (1995), “The most important factor controlling the generation of oil and gas is the hydrogen content of the organic matter (OM)”. Here, Hydrogen Index values vary between 62 and 428 mg HC/g TOC. Oxygen Index values range between 38 and 131 mg CO₂/g TOC. To illustrate the results HI and OI are plotted in the pseudo-van Krevelen diagram in Fig. 4. Samples 13/651, 13/696, 13/698 and 13/699 of the Buchenstein Fm. can be characterized as containing Type II kerogen. Their low PI values indicate either that these source rocks have expelled petroleum, or alternatively, they may result from weathering (Littke et al. 1991). The high S2 peaks show that burial temperature never reached the peak oil generating stage (Rullkötter et al. 1988).

Table 2 Rock–Eval pyrolysis and TOC data

Fig. 4

HI vs. OI plot of the Rock–Eval VI data (Buchenstein Fm. blue x, Bellerophon red x). The lines represent medians of the kerogens types I–III and are based on Espitalié et al. (1985)

All other samples can be characterized as bearing Type III kerogen, indicating a terrigenous origin of the organic matter. It should be noted that due to the analyses of whole rock samples, instead of pure kerogen samples, the OI values might be overestimated and the HI underestimated (Peters 1986).

T_max values range from 425 to 451 °C and basically agree with maturity information based on vitrinite reflectance data. Nevertheless, minor discrepancies are observed, e.g. samples 13/696 and 13/699 show low T_max values, although vitrinite reflectance values are at 0.7 %.

4.4 Thermal history modeling

The stratigraphy used as input for the simulation is shown in Table 3. It was compiled from Bosellini (1998), Bosellini et al. (2003) and Keim et al. (2001). The model includes 21 layers ranging in age from the Bozen Quartz Porphyry of the Athesian Volcanic Group to Quaternary sediments. Thicknesses were modified after Tscherny (2006) and the absolute ages are from Ogg et al. (2014) supplemented by data of Mundil et al. (1996, 2010). The relevant lithologies (Table 3) have been built with the PetroMod Lithology Editor and are based on field observations as well as descriptions of Bosellini (1998). For periods of uplift fixed thickness values for eroded strata are used with the exception of the Cretaceous/Tertiary erosion. The thickness of sediments eroded during Cretaceous/Early Tertiary was derived from modeling results. Short-term erosion events occurred during the Early Triassic and the Miocene. It should be noted that the timing of erosion of Triassic to Cretaceous formations is largely unknown and arbitrary values are shown in Table 3. Our model reveals the total eroded thickness only as it is based on one single erosional event.

Table 3 Stratigraphy used as fixed input for the simulation

For the conceptional model six different phases were initially defined: (1) Sedimentation from 267 to 244 Ma (Val Gardena SS to Werfen Fm.), (2) erosion of parts of the Werfen Fm. 244–242.8 Ma, (3) sedimentation from 242.8 to 155 Ma (Perez Fm. to Rosso Ammonitico Fm.), (4) erosion from 155 to 145 Ma (S. Cassiano Fm. to Ammonitico Rosso. completely eroded), (5) sedimentation of Puez Marls, Antruilles Fm. and Upper Cretaceous (145–65 Ma), and (6) uplift, Alpine Orogenesis and erosion from 65 Ma until present with contemporaneous sedimentation of thin Cenozoic sediments in parts of the area.

With respect to the thermal evolution of the basin a general heat flow of 65 mW/m² was chosen as the base of the model, representing an average global heat flow value. Moreover, the following four thermal periods were defined (Fig. 5): (1) Rifting of the continental crust with Ladinian tectono-magmatic event in the early Middle Triassic (max heat flow: 65 mW/m²), (2) opening of the Ligurian-Piemontese ocean and generation of oceanic crust in Middle and Upper Jurassic (max heat flow: 75 mW/m²), (3) consolidation phase with crustal thickening starting in Upper Jurassic (cooling), (4) since Eocene, crustal thickening due to Meso- and Neoalpine orogenesis.

Fig. 5

Paleo water depth (PWD), sediment water interface temperature (SWIT) and heat flow (HF) over time. In the heatflow curve, the rifting of the continental crust (1) and the opening of the Ligurian-Piemontese Ocean (2) are highlighted

Based on this scenario the heat flow (HF) vs. time trend shown in Fig. 5 was generated. Fission track data interpretation of Tscherny (2006) was used to link time and temperature data (max. temperatures 100–110 °C in the Cretaceous, rapid cooling in the Eocene). Values for the period of Ladinian volcanism are relatively low since all samples were taken in areas without volcanic influence. The paleo water depth curve (PWD) shows an assumed fluvial to shallow marine evolution from the beginning of the Middle Triassic to the Cretaceous. Sediment water interface temperature was influenced by palaeolatitude as well as water depth during deposition. Deeper water with cooler temperatures characterizes sedimentation of the Buchenstein and Wengen Formation. The minimum temperature in the Middle Triassic is due to the rapid drowning of the carbonate platforms from tropical surface temperatures to cold deep sea water (Bosellini et al. 2003). The thermal evolution of the basin fill is strongly controlled by rock properties such as thermal conductivity, heat capacity and radiogenic heat production. VR depth trends for all 1D-pseudowells were generated to study temperature history. Since the simulations of all pseudo-wells show similar trends, only the Gardena Valley model is described and discussed here in detail. Results of the sensitivity analyses are shown in Fig. 6. In a first scenario, a best fit case was achieved using the predefined heat flow curve and an optimum thickness for the Cretaceous erosion (Fig. 6a). The plot shows the VR data calculated according to Sweeney and Burnham (1990) as colored lines in comparison with the measured VR data as crosses. With increasing eroded thicknesses the calculated VR lines are shifting to higher VR values, based on a higher paleo overburden and therefore a greater burial depth. As a consequence of the non-constant heat flow curve, the lines are not completely parallel to each other. The best fit curve correlates with an eroded thickness of 1800 ± 200 m. This ‘best fit’ corresponds to a present day vitrinite reflectance depth gradient of 0.02 % VR_r/100 m, between the top of the Contrin Fm. and the Bellerophon Fm. It should be noted that there is a scatter in the measured vitrinite reflectance values. In such a case, the values showing low reflectance are used for calibration, i.e. the best-fit curve should fit especially the low reflectance values. This procedure takes into account that many samples contain resedimented vitrinites from erosion of older rocks, showing higher reflectances than autochthonous vitrinites.

Fig. 6

Results of sensitivity analyses of the Grödnertal model. A second model in the Frötschbach area shows same results. Simulated are the predefined HF curve (Fig. 5) with changing eroded thickness (a), changing eroded thickness at a fixed HF of 65 mW/m², held constantly over time (b) and changing constant HFs at a constant eroded thickness of 1200 m (c). The lines in the plot represent the calculated VR. The VR data used for calibration is indicated by crosses (+). Depth is referring to sea level

To illustrate the influence of different eroded thicknesses and different heat flows separately, scenarios b and c are shown in Fig. 6: Scenario b represents a model based on constant heat flow of 65 mW/m², which is close to the recent global mean value (Allen and Allen 2005). The lines of the different eroded thicknesses are parallel to each other (Fig. 6b) and shifted to higher VR data with increasing eroded thicknesses. A best fit line is achieved for a thickness of around 1000 m of eroded sediments. The eroded thickness is much lower than in case a, due to higher Cretaceous heat flows. Although a constant heat flow is highly unlikely in view of the complex and variable tectonic setting, scenario b shows that erosional thicknesses have to be treated cautiously.

Scenario c illustrates the use of different heat flows at a constant erosional thickness of 1200 m. Higher VRs are reached with increasing heat flows. Differences are increasing with increasing depth in this case (Fig. 6c).

In addition, an influence of higher heat flow values during the Permian has been analyzed: Even 120 mW/m², as maximum value for an assumed rifting (Allen and Allen 2005), during the complete period of the Permian would not increase the present day thermal maturity, as it is overprinted by later burial during the Cretaceous.

Figure 7a shows the burial plot of the Gardena Valley model, simulated with the predefined heat flow curve and an eroded thickness of 1800 m for the Cretaceous sediments. The thermal history of the sediments is displayed in this diagram as overlay. Conspicious events shown are the rapid drowning of the basin in the Middle Triassic with a subsequent basin shallowing at constant sedimentation (1) and two phases of major basin inversion (2, 3) aside from the Neogene inversion and uplift. Temperatures reach 158 °C at around 160 Ma before present, with the initiated opening of the Ligurian-Piedmontese ocean, leading to VR values of 1.28 % (2). In addition, the layers were thermally overprinted again 65 Ma before present at the point of deepest burial (160 °C, 1.32 % VR_r) triggered by the sedimentation of the Cretaceous layers.

Fig. 7

Burial plot with temperature history of the Gardena Valley model (a) and the vitrinite reflectance evolution of Bellerophon and Buchenstein Fm. Simulated with the predefined HF curve and the best fitting eroded thickness of 1800 m (see Fig. 6a)

The vitrinite reflectance vs. time trend (Fig. 7b) shows a clear correlation with the burial trend (Fig. 7a). As temperatures are increasing during times of burial, vitrinte reflectance increases. Due to the fact that the coalification process of vitrinites is irreversable their reflectance is not changing during times of uplift and a second phase of coalification will only be initiated by a second phase of burial provided that previous temperatures are reached or exceeded.

Previous simulations did not include internal radiogenic heat production within sedimentary rocks (e.g. Tscherny 2006). This aspect needs to be kept in mind for a comparison with the new data set of this study. For the Gardena Valley model one simulation was also performed without considering the influence of internal heat production. VR vs. depth trend is shifted to lower VR values, if the internal heat production is neglected. Thus, assumption of more overburden is needed to fit the calculated data with the calibration points of the field data. As a result, the Cretaceous erosion in the Gardena Valley model increases by additional 250–300 m or around 12 % of the total eroded thickness, if the radiogenic heat production is ignored.

4.5 Maturity

The model was calibrated by maturity data measured on samples collected from outcrops. Calibration was performed using VR data ("Appendix"), which were confirmed by T_max-values from Rock–Eval analysis. Due to the strong volcanic activities during Ladinian times in the area of the western Dolomites there is some scatter in the data observed (Fig. 6). Nevertheless, the results match largely with published data. As described by Tscherny et al. (2011), VR data for the Middle and Upper Triassic measured in the northern Gardertal near Badia by Balzas and Koncz (1999) range between 0.67–1.03 % and VRs measured by Zattin et al. (2003) range between 0.6 and 1.0 %. Tscherny et al. (2011) added own data in the range of 0.6–0.9 % VR. The results of the present study for the Buchenstein Fm. (0.61–0.76 % VR_r) agree more with the results of Tscherny (2006) (Buchenstein: 0.63–0.75 % VR_r) as compared to those by Zattin et al. (2003), who measured a value of 0.6 % VR_r for the Moena Fm. (see Fig. 2).

Reflectance data and sample locations were compared to those of Tscherny (2006). For the samples of the Buchenstein Fm. no regional trend of increasing maturity is observed (Fig. 8). Moreover, the comparison shows that the Buchenstein data of Tscherny (2006) do not indicate an increase in VR as suggested for other formations from north to south. Slightly increased or decreased VR values could also be explained by changes of sediment thicknesses deposited and by local differences with respect to the influence of tectonic events. For the Bellerophon Fm. it seems that a weak increase of VR occurs from west to east. This was suggested by Tscherny (2006) as well, based on his and additional literature data. Furthermore, Emmerich et al. (2008) analyzed sandstones of the Gröden Fm. at the Rosengarten, around 20 km south of the Frötschbach area, and classified VRs higher than 0.9 % VR_r as “not used: proximity to a volcanic dyke”. This threshold was confirmed in the present study.

Fig. 8

Regional distribution of VR values of the Buchenstein Formation. G is referring to this study and T to Tscherny (2006). Values in parentheses are regarded as ‘too high’, possibly influenced by local volcanism or measured resedimented vitrinites

As shown before, maturity levels based on vitrinite reflectance data are basically in good agreement with T_max values from Rock–Eval analysis. They vary between 425 and 451 °C. Tscherny (2006) reported T_max ranging from 420 to 465 °C. In addition, the samples of Tscherny (2006) represent, like the ones measured here, type II and type III kerogen.

4.6 Burial and temperature history

Summarizing the results from both regional models, an eroded thickness of 1800 ± 200 m of Cretaceous sediments was documented for the northwestern area of the Dolomites combined with a present day maturity gradient of 0.02 % VR_r/100 m. This erosion was initiated by the Alpine Orogenesis starting around 65 Ma and lasting until present day with most rapid uplift at Early Eocene and Late Miocene times. This eroded thickness adds to the eroded thickness of older layers. In the model, it is expected that the eroded Triassic and Jurassic layers were removed during the Late Jurassic (Table 3; Fig. 7a). For our Val Gardena model, we assumed an eroded thickness of pre-Cretaceous units of 500 m, which may be regarded as a maximum scenario. Even if this maximum scenario maturity (vitrinite reflectance) is overprinted by the much larger Tertiary erosional phase. This underlines that even the maximum possible case of Jurassic erosion is not influencing present-day maturity.

Tscherny et al. (2011) published similar results of 1700–2400 m for the Cretaceous erosion with a vitrinite reflectance depth gradient of 0.01–0.02 % VR_r per 100 m. These results were developed on the basis of relatively low heat flows: A peak of 55 mW/m² was assigned for the rifting at Jurassic times (160 Ma) and a present day value of 30 mW/m². In contrast, this work used 75 mW/m² for the Jurassic rifting event and 41 mW/m² for the present day value. Based on sensitivity analyses with constant erosion, this slightly higher HF value seems to be more realistic. Heat flows at maximum burial, during the latest Cretaceous, show similar differences with 35 mW/m² in Tscherny et al. (2011) and 46 mW/m² in this work. It should be noted, that there are varying values published for the Jurassic heat flows as some authors even state very high values of up to 100 mW/m² (e.g. Zattin et al. 2003).

Former investigations by Tscherny (2006, without radiogenic heat production and with a present day HF of 41 mW/m²) result in maximum eroded thicknesses for the Upper Cretaceous of 2000 m (original value of 2300 m prior to correction of internal heat production) and a maturity trend of 0.02–0.03 % VR_r/100 m. Carminati et al. (2010) deduced an erosional thickness of 2200 m for Cretaceous to Paleocene strata, applying relatively high heat flows of 85–105 mW/m² during Middle Jurassic times. Thus, their results confirm the estimates of paleo thicknesses presented here. The values of the recent HF are the same in this and in Tscherny (2011, 41 mW/m²). Ladinian volcanism did not influence maturity of the deepest Triassic and Permian layers or had only local influence (Tscherny 2006).

At this point it should be mentioned that a general increase of maturity could have been influenced by the assumption of overthrusting and later erosion of this tectonical load as well. Doglioni (1985) documented that there are minor overthrusts of Miocene age in the Dolomites. Orogenic loading may have resulted in burial of underlying sedimentary rocks, temperature increase and maturation similar to burial by additional sediments.

5 Conclusions

New vitrinite reflectance data supplemented by solid bitumen reflectance and Rock–Eval data provide important information on thermal maturity of the northwestern Dolomites. Almost all rocks are within the ‘oil window’, but have not passed the peak oil generation stage (0.9 % VR_r). Exceptions are local areas influenced by volcanism.

The vitrinite reflectance data was used to calibrate temperature and burial history using PetroMod software. Based on these numerical models an eroded thickness of 1800 ± 200 m appears likely for the Cretaceous layers of the study area. The maturity of the Buchenstein Fm. ranges from 0.61 to 0.76 % VR_r. This requires a substantial burial and a thicker cover as present today. Times of deepest burial and maximum temperatures presumably correspond to Late Cretaceous/Early Paleocene times. The present day maturity gradient (0.02 % VR_r/100 m) indicates quite a low paleo heat flow situation. After basin inversion was initiated around 65 Ma, the original overburden was eroded during a process of basin uplift. This process mainly took place during the Alpine Orogenesis in Eocene and Miocene times. Burial histories indicate that the influence of deepest burial at around 65 Ma with respect to maturity of Permian and Early Triassic strata overwhelmed earlier temperature events such as high heat flows during the Ladinian.

Nevertheless, it cannot be excluded that the increase of maturity was initiated by overthrusting and later erosion of the overthrusted layer as orogenic loading may have caused the same signal of deepest burial as loading by additional sediments.

Appendix

See Table 4.

Table 4 Vitrinite reflectance data